This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of mobility control slugs.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
The injected aqueous medium employed in waterflooding may be simply field brine or surface water or it may contain various other materials. For example, the injected water may contain surface-active agents which reduce the interfacial tension between the injected water and oil or may contain various agents such as alkaline agents which react with injected or indigenous material in the reservoir oil to produce surface-active agents within the reservoir. The injected aqueous flooding medium may also comprise water and oil emulsions with the water being either the dispersed or continuous phase. For example, U.S. Pat. No. 3,261,399 to Coppel discloses a waterflooding process in which water-in-oil and oil-in-water emulsions are successively injected into the formation.
One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous displacing medium; that is, the injected displacing medium tends to channel through certain portions of the reservoir as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors such as differences in the mobilities of the injected displacing liquids and the displaced reservoir oil and permeability variations within the reservoir which encourage preferential flow through some portion of the reservoir at the expense of other portions.
Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing medium and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected displacing medium in order to increase the viscosity thereof and thus decreasing its mobility to a value equal to or less than the mobility of the displaced reservoir oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Mobility control by this procedure may be accomplished by employing a so-called "square" mobility control slug in which the concentration of the thickening agent is relatively constant throughout. Other procedures have involved the use of a so-called "graded" mobility control slug. For example, in a paper by W. R. Foster entitled "A Low Tension Waterflooding Process", JOURNAL OF PETROLEUM TECHNOLOGY, Vol. 25, February, 1973, pp. 205-210, there is disclosed a low tension waterflood process which involves the sequential injection of a protective slug, a surfactant slug, and a mobility control slug. The surfactant slug comprises an aqueous solution of petroleum sulfonates. The mobility control slug comprises water thickened with a viscosifier such as a water-soluble biopolymer in a graded decreasing concentration in order to provide an initial viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. Thereafter, a driving fluid such as produced field brine is injected in order to carry the process to conclusion.
The aforementioned patent to Coppel also discloses the use of a graded viscosity mobility control slug injected in various sequences. For example, Coppel proposes following an oil-external emulsion with a first water-external emulsion and following this emulsion with a second water-external emulsion in which the viscosity is incrementally reduced to that of the subsequently injected driving water. Alternatively, Coppel suggests that the water-external emulsions may be replaced by an aqueous polymer slug in which the polymer concentration, and thus the slug viscosity, may be progressively reduced.
A paper by Mungan, N., "Programmed Mobility Control in Polymer Floods", The 3rd Symposium of the Assn. de Recherche sur les Techniques de Forage et de Production, Pau, France (June, 1968), presents a mobility control configuration designed to offset the limitations presented by adsorption of the polymer from the mobility control slug. The author notes that the polymer concentration decreases at the leading edge of the mobility control slug because of adsorption with the result that the aqueous medium moves ahead of the polymer. He further concludes that at the trailing edge of the slug little or no polymer adsorption takes place, thus causing an unfavorable mobility ratio between the polymer slug and the following water with the result that the water tends to miscibly finger through the polymer slug. To offset the effects of these adsorption phenomena, Mungan proposes to first inject a 4 percent slug of a 0.2 percent polymer solution and to follow this with a graded polymer slug in which the polymer concentration is decreased according to an exponential function.